1. Field of the Invention
The present invention relates to a photodetector circuit for detecting light, such as visible light or infrared light, and more particularly, to a photodetector circuit having low current consumption compared with a conventional one.
2. Description of the Related Art
Recent electronic devices provided with a function unnecessary under low ambient light conditions, such as time striking in a wall clock, or a function necessary under low ambient light conditions, such as a backlight of a mobile phone, have detected ambient light conditions by means of a built-in photodetector circuit so that the above-mentioned function may be enabled only when needed, to thereby achieve low power consumption. However, due to the recent demand for still lower power consumption, there arises a need to reduce power consumption of the built-in photodetector circuit as well.
FIG. 5 illustrates a circuit for controlling whether or not to strike the time in a wall clock by means of a conventional photodetector circuit. As illustrated in FIG. 5, the circuit has the following configuration. A photodiode 120 has an N-type terminal connected to a positive power supply terminal VDD, and a P-type terminal connected to a first electrode of a current limiting resistor 501. A transistor 510 has a collector and a base which are connected to a second electrode of the current limiting resistor 501 and a base of a transistor 511. The transistor 510 has an emitter connected to a reference power supply terminal GND. The transistor 511 has a collector connected to a first judgment node N1, and an emitter connected to the reference power supply terminal GND. A transistor 520 has a collector connected to the first judgment node N1, and a base connected to a collector of a transistor 521 and a first electrode of a reference resistor 502 together with a base of the transistor 521. The transistor 520 has an emitter connected to a second electrode of a switch 530 and an emitter of the transistor 521. The switch 530 has a first electrode connected to the positive power supply terminal VDD. The reference resistor 502 has a second electrode connected to the reference power supply terminal GND. A P-channel metal oxide semiconductor (MOS) transistor 104 has a source connected to the positive power supply terminal VDD, a gate connected to the first judgment node N1, and a drain connected to an output node N2. An N-channel MOS transistor 107 has a source connected to the reference power supply terminal GND, a gate connected to the first judgment node N1, and a drain connected to the output node N2. The output node N2 is connected to a clock control unit 540. Although not illustrated, the positive power supply terminal VDD is supplied with a positive voltage from a power source, while the reference power supply terminal GND is supplied with a zero-volt voltage from the power source.
The conventional photodetector circuit is configured as described above, and operates as follows to detect light while the switch 530 is turned ON.
The photodiode 120 generates a photocurrent that is proportional to brightness, and the photocurrent is mirrored into the transistor 511 via the transistor 510. A reference current flowing through the reference resistor 502 is mirrored into the transistor 520 via the transistor 521. Accordingly, the current flowing through the transistor 511 based on the photocurrent is compared at the first judgment node N1 with the current flowing through the transistor 520 based on the reference current. Under high ambient light conditions, the current flowing through the transistor 511 based on the photocurrent is larger in amount than the current flowing through the transistor 520 based on the reference current. Therefore, the first judgment node N1 is Low while the output node N2 is High, and hence the clock control unit 540 enables a time striking function in response to the signal of High. On the other hand, under low ambient light conditions, the current flowing through the transistor 511 based on the photocurrent is smaller in amount than the current flowing through the transistor 520 based on the reference current. Therefore, the first judgment node N1 is High while the output node N2 is Low, and hence the clock control unit 540 disables the time striking function in response to the signal of Low (see, for example, JP 2006-287658 A).
In the conventional photodetector circuit, the switch 530 is turned ON for a minimum time period when a photodetection operation needs to be performed, to thereby achieve low current consumption. However, a pulse circuit needs to be provided to perform intermittent control on the switch 530, which leads to a problem of an increased circuit scale. In addition, because a consumption current of the pulse circuit flows all the time, there is another problem that the consumption current of the pulse circuit cannot be reduced. Further, in the conventional photodetector circuit, the photocurrent of the photodiode 120 is not cut off by the switch 530, which leads to still another problem that the photocurrent is consumed all the time under high ambient light conditions.